1. Field of the Invention
The present invention relates to a battery voltage monitoring apparatus for detecting a battery voltage of a power supply apparatus composed of a plurality of secondary batteries connected in series, and particularly to a battery voltage monitoring apparatus for detecting a disconnection of a signal line when the line is disconnected while measuring a voltage.
2. Description of the Related Art
An electric car and a hybrid car are known as an environmental friendly vehicle. In the electric and hybrid cars, a motor is used as a driving source to drive vehicles. An electrifiable secondary battery is connected to the motor. A direct-current voltage obtained from the secondary battery is converted into an alternating voltage to drive the motor. As the secondary battery requires high voltage, the secondary battery is usually formed as an assembled battery having a plurality of battery cells connected in series.
A plurality of voltage sensors are used to detect a voltage of each battery cell in the assembled battery. The voltage sensors are modularized by appropriate number. If the number of battery cells connected in series is large as in the electric car, a plurality of the modules of the voltage sensors are prepared to be connected in series. An apparatus for monitoring voltages of such assembled battery is disclosed in Japanese Unexamined Patent Application Publication No. 2003-208927, No. 2003-111284, and No. 2005-117780. An apparatus to monitor voltages of the assembled batteries is described hereinafter in detail. For, example, the monitoring apparatus has a plurality of semiconductor apparatuses (modules of the voltage sensors, which are ICs) that are connected in series. Each semiconductor apparatus (IC) have the plurality of battery sensors.
FIG. 13 is a schematic view showing a conventional voltage monitoring apparatus 100. As shown in FIG. 13, one IC is able to detect voltages of four battery cells. Each input terminal is connected to battery cells C11 to C18 via lines for voltage measurement L11 to L19. The IC11 of FIG. 13 operates with a positive electrode (node N11) of the battery cell C11 as a power supply potential, and a negative electrode (positive electrode of the battery cell C15, which is node N12) of the battery cell C14 as a ground potential. IC12 is connected in series with IC11. IC12 operates with a positive electrode (node N12) of the battery cell C15 as the power supply potential, and a negative electrode of the battery cell C18 (node N13) as a ground potential. Each of the ICs detects an overvoltage or a low-voltage condition of the battery cells to monitor, and output an overvoltage or a low-voltage detection signal. In such apparatus having a plurality of ICs or the like connected in series to monitor voltages of assembled batteries, there is a following problem if a line such as the line L15 is disconnected, where the line is a connecting line between ICs.
If the line L15 of FIG. 13 is disconnected, the voltage is not supplied to the node N14 from the battery cell. Thus the voltage of the node N14 increases or decreases. With the increased voltage of node N14, potential differences (voltage) increase between the terminals of IC12, which are V15 and V16, and between the power supply and the ground. With the decreased voltage of the node N14, potential differences (voltage) increase between the terminals of IC11, which are V14 and VSS, and between the power supply and the ground.
It has now been discovered that as a result of the increased potential difference between terminals of ICs or between the power supply and ground due to the disconnection of the line, the potential difference exceeds the voltage between terminals and the power supply voltage, thus possibly destroying the ICs as the modules for voltage sensor.